Contact printing and projection printing are known masking techniques used in the fabrication of semiconductor devices. Projection printing offers the advantage of avoiding contact between the mask and a photoresist-coated semiconductor wafer. A known projection printer apparatus is described in U.S. Pat. No. 3,819,265, issued to M. Feldman and M. C. King on June 25, 1974 and assigned to the assignee hereof. In such a known apparatus, the mask and the photoresist-coated semiconductor wafer are mounted on a common movable translation table. A small portion of the mask is imaged on the wafer by a high resolution optical system. The latter typically consists of an even number of imaging lenses, a field lens, and an odd number of mirrors in order to achieve proper image orientation and compensate for the effects of image inversion. Although this known scanning projection apparatus is acceptable for certain applications, it requires a large number of lenses and mirrors resulting in a relatively complex and expensive system. Furthermore, the spacing between corresponding points on the mask and wafer is such that scanning tolerances are small, thus requiring an expensive scan table.
A simple and economical one-to-one imaging optical system is described in an article by J. Dyson entitled "Unit Magnification Optical System without Seidel Aberrations" published in Journal of the Optical Society of America, Volume 49, No. 7, July 1959, pages 713-716. This known Dyson system consists of two components, namely, a concave spherical mirror of radius R, and a thick plano-convex lens of radius r, index n and thickness equal to r. The centers of curvature of both spherical surfaces are substantially coincident, and r is chosen so that parallel rays incident on the plano surface are focused on the mirror surface, i.e., EQU 1/(R-r)=(n-1) (1/-r) (1)
or EQU r/R=(n-1)/n (2)
In this known system, object and image surfaces lie on or close to the plane face of the lens. Furthermore, object and image are of opposite directions rendering such a system inapplicable by itself to scanning photolithographic projection printers.
A modified Dyson optical system is described in a paper by C. G. Wynne entitled "A Unit-Power Telescope for Projection Copying" in a book entitled Optical Instruments and Techniques, pages 429-434, edited by J. H. Dickson and published by Oriel Press, Ltd., England, 1970. This known optical arrangement utilizes a beam splitter to separate the object and image planes of Dyson. The foregoing is achieved by forming a beam-splitting cube from a section of the plano-convex lens of the system resulting in an image plane at 90.degree. from the object plane. A similar optical printing system is described in U.S. Pat. No. 2,231,378, issued on Feb. 11, 1941 to H. Becker et al. These two known printing systems require complex, heavy and bulky optics. Moreover, the known systems cannot be used in a compact "in-line" scanning projection printer, i.e., in a printing system wherein object and image have the same direction and are located in parallel planes substantially in alignment on top of each other.
In U.S. Pat. No. 3,054,326, issued Sept. 18, 1962 to H. Giesecke, a projecting device for photolithography is described comprising a roof prism, a mirror and an objective positioned therebetween. In this known projecting device, the roof prism is mounted within a cardanic suspension such that it is possible to modify the thickness of the image lines by tumbling the roof prism. Although in such a known device the object and image planes are parallel, the overall construction and accuracy of the device are such that it is inapplicable and ineffective for highly accurate semiconductor scanning projection printers. Furthermore, the objective located between the prism and the mirror comprises a plurality of symmetrically arranged lenses, the latter adding complexity and cost to the overall device.